Gate valves of the above type are known which comprise a gate coated on the outside with rubber except for the guide areas. The gate is often made of cast iron with the result that corrosion problems may arise. As a consequence thereof gates entirely coated with rubber are commonly used today. However, a disadvantage of coating the guide areas with soft rubber is an increased friction and wearing off of said guide areas in some situations, such as when the valve is used for extremely high pressures. The increased friction causes in turn an increase in the stem force for opening and closing the valve.
U.S. Pat. No. 4,532,957 discloses furthermore a gate valve with a gate comprising a gate core. On each side the gate core comprises a groove wherein a plastic insert of a harder material is placed. The plastic insert forms a guide groove for mating guide rails inside the valve body. The entire outer surface of the valve gate except for the guide grooves is coated with a softer rubber material.
The manufacture of such a gate involves the steps of first manufacturing the inserts in a separate mould followed by placing the inserts in the grooves inside the gate core, said gate core subsequently being coated with rubber in a second mould.
European printed accepted specification No. 01 46 112 discloses furthermore a valve gate coated on the entire outer surface with a softer rubber-elastic material and provided with a U-shaped harder plastic insert on each side. The plastic insert forms a guide groove for guide rails in the body and is embedded in the rubber-elastic material, i.e. it is encapsulated by said material except for in the guide groove. Also the manufacture of such a gate involves the steps of manufacturing the inserts in a separate mould followed by a placing thereof together with the gate core in a second mould so as to provide the rubber coating.